1. Field of the Invention
The present invention relates to a variable-frequency oscillation circuit whose output frequency can be varied by digital control, and a digitally controlled oscillator employing the variable-frequency oscillation circuit to control the period of a pulse signal.
2. Description of the Prior Art
A voltage-controlled oscillator (hereinafter abbreviated as "VCO") whose output frequency can be controlled by an analog control voltage has been used as a variable-frequency oscillator for a phase-locked loop (hereinafter abbreviated as "PLL") in communications equipment or motor controllers.
The center frequency of the VCO is dependent on a specific resistor and a specific capacitor. Therefore, the resistor and the capacitor must be changed when the center frequency needs to be changed, and the resistor and the capacitor need fine adjustment to determine the center frequency accurately.
A digital control system employing a microcomputer or the like has been used for advanced control. However, digital control system incorporating a conventional VCO needs an A/D converter to translate digital control signals provided by the microcomputer into proportional analog signals, thereby requiring a complex circuit configuration and increasing the cost of the digital control system.
A known variable-frequency oscillation circuit capable of directly controlling its output frequency by a digital control signal divides the internal clock of a microcomputer or the like for triggering. However, since the conventional variable-frequency oscillation circuit capable of being controlled by digital control signals divides a clock signal to provide an output signal of a desired frequency, the frequency of the output signal is a low frequency on the order of 100 kHz or below. Accordingly, it has been impossible to apply the conventional variable-frequency oscillation circuit to communications equipment and motor controllers which deal with signals of frequencies in the range of several hundreds of KHz to several tens of MHz.